1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having bipolar and MOS transistors on one semiconductor substrate.
2. Description of the Related Art
In the formation of P- and N-channel MOS and bipolar transistors on a common substrate, an internal base region of the bipolar transistor is formed by the same step as a channel ion implantation step for the MOS transistor and, subsequent to forming the gate electrode of the MOS transistor, an external base region of the bipolar transistor is formed simultaneously with the formation of the source and drain regions of the P-channel MOS transistor.
In this conventional method, it is possible to simultaneously form the emitter electrode of the bipolar transistor and gate electrode of the MOS transistor. In the formation of a microminiaturized MOS transistor of high reliability it is required that a gate oxide film be formed subsequent to the channel ion implantation step and, since the formation of the gate oxide film of the MOS transistor is formed after the formation of an internal base region of the bipolar transistor, the depth xj of the internal base region will be increased due to an oxidation enhanced diffusion during the gate oxidation step. This causes an increase in the base width of the internal base region, largely degrading the characteristic of bipolar transistors. In the conventional method, it is not possible to form a high performance bipolar transistor because the internal base region of the bipolar transistor is formed at the same step as the channel ion implantation step for MOS transistors.
It is particularly important to form a gate oxide film as a thin one in the formation of microminiaturized MOS transistors. In the formation of a thin gate oxide film and hence a MOS capacitor of high reliability, ion implantation cannot be carried out subsequent to forming the gate oxide film. Since, therefore, a dummy gate oxide film is formed prior to the channel ion implantation step and then that channel ion implantation is carried out, a gate oxide film needs to be formed after the removal of the dummy gate film. Upon implantation of ions into the internal base in the same way as the conventional step, the internal base width will be made broader due to the OED (oxidation enhanced diffusion), so that the characteristics of bipolar transistors are greatly deteriorated.